Question

Alternating current cannot be measured by dc ammeter because, (a) ac cannot pass through dc ammeter (b) Average value of complete cycle is zero (c) ac is virtual (d) ac changes its direction

Short answer

The correct answer to the question, "Alternating current cannot be measured by a DC ammeter because" is option (b): Average value of the complete cycle is zero. The DC ammeter measures the average current, and since the average value of an AC waveform over a complete cycle is zero, the DC ammeter will not provide an accurate measurement of alternating current.

Step-by-step solution

Understand AC and DC current

Alternating current (AC) is an electrical current that changes its direction periodically, whereas direct current (DC) only flows in one direction. AC is commonly used for power transmission and distribution, while DC is employed in batteries, electronic devices, and other applications where a constant voltage is required.

Understand how a DC ammeter works

A DC ammeter is an instrument designed to measure the flow of direct current in a circuit. It operates on the principle of measuring the voltage drop across a known resistance (shunt resistor) proportional to the current flow. Since a DC ammeter is specifically designed to measure direct current, it cannot be used to accurately measure an alternating current.

Analyze each option

(a) ac cannot pass through dc ammeter: This statement is not entirely accurate. AC current can pass through a DC ammeter; however, the measurement will not be correct due to the alternating nature of AC current. (b) Average value of complete cycle is zero: This is the correct answer. The average value of an AC waveform over a complete cycle is zero, as the positive and negative portions of the waveform cancel out each other. A DC ammeter mainly measures the average current, which will be zero for AC. (c) ac is virtual: This statement is incorrect. AC is not virtual; it is a real form of electrical current used in many applications, such as power transmission and distribution. (d) ac changes its direction: While this statement is true, it doesn't explain why a DC ammeter is unable to measure AC current. The main reason why AC cannot be measured by a DC ammeter is due to the averaging effect on a complete cycle, not simply because it changes direction.

Conclusion

The correct answer to the question, "Alternating current cannot be measured by dc ammeter because" is option (b): Average value of complete cycle is zero. The DC ammeter measures the average current, and since the average value of an AC waveform over a complete cycle is zero, the DC ammeter will not provide an accurate measurement of alternating current.

Key concepts

DC Ammeter

A DC ammeter is a device specifically designed for measuring the flow of direct current (DC) in an electrical circuit. Unlike alternating current (AC), which changes direction periodically, DC flows in a single, unchanging direction. A DC ammeter measures current by monitoring the voltage drop across a known resistor, commonly referred to as a shunt resistor. This measurement is directly proportional to the current flowing through the circuit.

This device is crucial for applications where maintaining a constant current is essential, such as in battery-powered devices. However, because a DC ammeter measures the average current flowing through a circuit, it can only accurately measure current that does not change direction, which is why it falls short when trying to measure alternating currents.

Waveform Cycle

A waveform cycle is a complete oscillation of an alternating current or voltage. When we talk about AC, we often refer to its waveform, typically sinusoidal in shape. One full waveform cycle represents the entire passage of both the positive and negative halves of the AC waveform.

In a sinusoidal waveform, this cycle is characterized by moving from a zero point, reaching a positive peak, returning through zero, reaching a negative peak, and finally back to zero. This entire sequence constitutes one cycle. Importantly, because the positive and negative portions of the waveform cancel out over one complete cycle, the average value of a sinusoidal AC waveform over a full cycle is zero. This concept is crucial when understanding the limitations of using a DC ammeter to measure AC.

Current Measurement

Current measurement is a fundamental aspect of understanding and working with electrical circuits. When measuring electrical current, it is important to know whether the current is alternating (AC) or direct (DC), as each requires different instruments for accurate results. A DC ammeter measures the average direct current flowing through a circuit, which is consistent and unidirectional.

• For DC: We use devices like DC ammeters to measure steady flows without periodic changes in direction.
• For AC: Different instruments, such as AC ammeters or multimeters set to AC current, are used as they can accurately account for the changing nature of the waveform.

Understanding this distinction is important because the average value of an AC current over a cycle is zero, which makes a DC ammeter unsuitable for these measurements.

Electrical Current

Electrical current is the flow of electric charge around a circuit. There are two primary types of electrical current: alternating current (AC) and direct current (DC).

• AC (Alternating Current): This type of current changes its direction periodically and is the form of electricity most commonly delivered to businesses and homes.
• DC (Direct Current): Flows in a single direction, used in battery-powered devices and other applications that require a steady voltage supply.

Understanding these types of current is crucial for selecting the appropriate instruments for measurement and application. AC and DC are used in different contexts due to their unique characteristics, and the method of measurement varies as a result. DC is steady and non-fluctuating, while AC fluctuates regularly, which necessitates using different measurement devices for each type.